Numerical simulation of gas dynamics in the high-pressure gas atomization process

High pressure gas atomization (HPGA) is an effective method for producing high yields of ultrafine metal and alloy powders. The salient gas flow features during such a complicated process must be recognized and understood before full control can be maintained over the process dynamics. In this investigation, the turbulent, compressible Navier-Stokes equations were solved for the gas-only flow in the vicinity of the melt tip within an HPGA atomizer. The influence of operating pressure and melt tip geometry on the HPGA gas flow field and melt aspiration condition were then examined.; The numerical results gained in this study have characterized a recirculation region attached to the melt tip base and a shear layer along the interface between the supersonic expanding flow and subsonic recirculating flow at all operating pressures and a normal shock downstream of the recirculation region at high atomizing pressures. These predominant flow features are in good agreement with published flow visualization results, such as Schlieren photographs and hydraulic analogy with a water table. The recirculation region is seen to experience a transition from an open wake condition to a closed wake condition. During the closed wake operation, the melt tip base center pressure ratio no longer decreases but instead remains relatively constant when the operating pressure is increased.; Different HPGA nozzle and melt tip designs have significant influence on the gas dynamics and melt aspiration in the atomization process as manifested by the numerical results and experimental evidence. As a consequence, a longer tip protrusion, a smaller tip taper angle, a smaller gas jet apex angle and a closer fit of the tip frustum surface to the innermost boundary of the gas jet all encourage a subambient or near-ambient melt tip base pressure, which provides a stable atomization.; A 3-D model was established for a discrete jet HPGA atomizer, which reinforces the axisymmetric results. Generally, the 3-D model predicts lower gas flow field properties, such as the Maximum Mach number within the whole gas flow field and recirculation length, relative to the axisymmetric model.